1. Field of Invention
The present invention relates to a semiconductor process. More particularly, the present invention relates to an ion sampling system for wafer and a sampling method thereof.
2. Description of Related Art
In a fabrication process for a semiconductor wafer, contamination of the wafer surface is a serious problem. In general, contaminants that adhere on the wafer surface include organic compounds, metal impurities and ion contaminants, wherein these contaminants adversely affect the subsequent process of the products. For example, these contaminants may induce leakage current at the p-n junction of the device, reduce the life time of some carriers, lower the breakdown voltage of the gate oxide layer, cause erosion to the metal line, and other related problems. The lowering of the quality and the reliability of the semiconductor device is thus resulted. Further, the semiconductor device may even become ineffective. Therefore, during the fabrication of a semiconductor device, it is important to detect the amount of contaminants on the wafer surface.
During the detection process for contaminants on a wafer surface, the sampling procedure for contaminants on the wafer surface is the most important step. As an example, organic compound contaminants are sampled with a SWA (Silicon Wafer Analyzer) sampling instrument, followed by using a gas chromatography mass spectrometer (GC-MS) to detect the amount of the organic compound contaminants. The metal impurity contaminants are sampled with a WSPS (Wafer Surface Preparation System) instrument, followed by analyzing with an inductively coupled plasma mass spectrometer to determine the amount of the metal contaminants. The amount of the ion contaminants is determined by submerging the wafer in a two liter pure water for a period of time to transfer the ions from the wafer surface in the water. Ion chromatography is then used to concentrate and analyze the amount of the ion contaminants.
In accordance to the above wafer contaminant detection method, instrumentation is used to sample organic compounds and metal impurities. Further, the sampling of the organic compounds and the metal impurities is only directed to a single surface of the wafer because the fabrication of a semiconductor device is mostly conducted on one surface of the wafer, and only the presence of contaminants on that particular surface is relevant. Therefore, the detection accuracy for contaminants of organic compounds and metal impurities is higher. Since the sampling of ion contaminants is accomplished by submerging the wafer in two liters of pure water, ions on both sides of the wafer are being transferred in the water. The detection accuracy of ion contaminants is thereby questionable.
Further, since a large quantity of water is used to remove the ions from the wafer surface, the ion concentration in the water is very low. Therefore, more time is required for and is being wasted on concentrating the ion solutions. Further, the above procedure is conducted in an open environment, not only the wafer gets contaminated during the period of fabrication, it can be further contaminated during the sampling procedure. The contaminants in the environment could contaminate the pure water during the sampling procedure to affect the accuracy of the detection result for the ion contaminants.